The hydrodechlorination of 2,4-DCP was a stepwise process with 2-CP and 4-CP as partially intermediate products as shown Fig.1, they were further converted to phenol. Two molecules of hydrochloric acid also formed which act as a poison to the catalyst. There was no indication of cyclohexanone and cyclohexanol in the analytical results of GC. Compared with 2,4-DCP, phenol is less toxic and useful as an intermediate in the manufacturing of different chemicals.
Figure : The hydrodechlorination process of 2,4-DCP over Pd/C catalyst at 30°C
In the literature 2-CP was the only dechlorinated intermediate in the HDC of 2,4-DCP in liquid system[12-16]. In this study, it could be seen that when different bases used 2-CP and 4-CP formed as intermediate products. This trend was not coincident with the results reported in literature [12-16].
Effect of base on hydrodechlorination of 2,4-Dichlorophenol
During hydrodechlorination of of 2,4-dichlorophenol, HCl produced as a by-product which reduce the activity of the catalyst. Therefore a base was used to neutralize the HCl. In this study different bases such as Ammonium hydroxide, Sodium hydroxide, Triethylamine and Hydrazine used to neutralize the HCl from the reaction mixture.
Amonium hydroxide
When ammonium hydroxide (NH4OH) used as a base, the dechlorination of 2,4-DCP completed within 80 min. 2-CP and 4-CP formed as intermediate products and these were further converted to phenol. From table it was clearly observed that when amount of NH4OH increased, the amount of 2-CP decreased but the amount of 4-CP remain almost constant. The dechlorination time was remaining constant as the amount of NH4OH increased.
Table 1: The hydrodechlorination of of 2,4-DCP over 5% Pd/C catalyst with Ammonium hydroxide (NH4OH)as a base
Amount of base(mmol)
Time(min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
3
0
15
30
45
60
75
90
100
81.49
59.39
37.84
6.63
1.23
0
0
8.13
10.91
5.32
2.35
1.21
-
0
0.68
1.03
0.38
0.21
-
-
0
9.70
28.67
56.46
90.81
97.56
100
6
0
15
30
45
60
75
90
100
81.39
58.32
36.21
7.65
0.80
0
0
6.69
8.37
5.31
2.51
1.26
-
0
2.93
1.31
0.42
0.24
-
-
0
8.89
32.00
58.06
89.6
97.94
100
9
0
15
30
45
60
75
90
100
80.37
56.64
38.52
8.32
0.48
0
0
3.73
6.78
3.32
2.10
1.01
-
0
0.47
1.09
0.54
0.12
-
-
0
9.70
28.67
56.46
90.81
97.56
100
12
0
15
30
45
60
75
90
100
83.82
58.38
35.32
6.97
0.26
-
0
2.42
4.64
2.09
1.83
0.89
-
0
0.91
1.32
0.53
0.19
-
-
0
12.85
35.66
62.06
91.01
98.85
100
Sodium Hydroxide
When Sodium Hydroxide (NaOH) used as a base, the dechlorination of 2,4-DCP completed within 60 min. 2-CP and 4-CP formed as intermediate products and these were further converted to phenol. From table it was clearly observed that when amount of NaOH increased, the amount of 2-CP and 4-CP remains almost. The dechlorination time was remaining constant as the amount of NaOH increased.
Table 2: The hydrodechlorination of of 2,4-DCP over 5% Pd/C catalyst with Sodium hydroxide (NaOH)as a base
Amount of base(mmol)
Time(min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
3
0
15
30
45
60
100
70.58
37.45
12.72
0
0
3.31
3.43
3.32
-
0
2.97
2.23
-
-
0
23.14
58.80
83.97
100
6
0
15
30
45
60
100
71.02
35.69
13.85
0
0
6.43
2.52
1.81
-
0
1.69
0.31
-
-
0
20.86
61.48
84.34
100
9
0
15
30
45
60
100
68.17
34.87
14.56
0
0
6.59
3.39
1.93
-
0
1.35
0.39
-
-
0
23.89
61.35
83.51
100
12
0
15
30
45
60
100
71.65
36.05
14.78
0
0
5.93
3.18
2.00
-
0
1.56
0.57
-
-
0
20.86
60.19
83.22
100
Triethylamine
When triethylamine (Et3N) used as a base, the dechlorination of 2,4-DCP completed within 48 min. 2-CP and 4-CP formed as intermediate products and these were further converted to phenol. From table it was clearly observed that when amount of Et3N increased, the amount of 2-CP and 4-CP decresed. The dechlorination time was remaining constant as the amount of Et3N increased.
Table 3: The hydrodechlorination of of 2,4-DCP over 5% Pd/C catalyst with Triethylamine (Et3N))as a base
Amount of base(mmol)
Time (min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
3
0
10
20
30
40
50
100
73.71
52.23
15.71
8.32
0
0
13.22
11.70
5.75
4.25
-
0
1.01
2.91
0.31
0.22
-
0
12.07
33.16
78.24
87.20
100
6
0
10
20
30
40
50
100
74.44
41.95
12.72
2.78
0
0
8.01
14.17
3.32
2.72
-
0
0.24
0.36
-
-
-
0
17.31
43.52
83.97
93.57
100
9
0
10
20
30
40
50
100
70.57
43.41
15.53
7.87
100
0
17.73
15.49
8.99
2.93
-
0
0.29
0.68
0.17
-
-
0
11.40
40.42
75.58
89.2
100
12
0
10
20
30
40
50
100
72.19
40.37
14.78
6.01
0
0
6.48
3.14
2.00
0.46
-
0
0.98
0.11
-
-
-
0
20.35
56.37
83.22
93.57
100
Hydrazine
When 3 mmol hydrazine (N2H4) used as a base, the dechlorination of 2,4-DCP completed within 43 min. 2-CP and 4-CP formed as intermediate products and these were further converted to phenol. From table it was clearly observed that more 4-CP formed than that of 2-CP. Dechlorination time decreased when the amount of N2H4 increased.
Table 4: The hydrodechlorination of of 2,4-DCP over 5% Pd/C catalyst with Hydrazine (N2H4)as a base
Amount of base(mmol)
Time (min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
3
0
10
20
30
40
50
100
73.80
44.15
14.99
0.80
0
0
4.90
3.84
3.56
1.50
-
0
8.27
9.20
10.63
4.30
-
0
13.03
42.81
70.82
93.40
100
6
0
10
20
30
40
100
71.89
37.45
7.36
0
0
2.63
2.23
1.25
-
0
6.48
3.43
2.73
-
0
19.00
58.80
88.66
100
9
0
10
20
30
40
100
67.05
34.01
3.65
0
0
0.46
4.93
2.87
-
0
6.48
6.32
1.55
-
0
29.94
54.74
91.93
100
12
0
10
20
30
40
100
58.83
31.01
1.26
0
0
2.58
5.48
1.51
-
0
4.97
7.11
2.08
-
0
33.62
56.40
95.16
100
From the above discussion it was seen that dechlorination time of 2,4-DCP follow the order of hydrazine > triethylamine > sodium hydroxide > ammonium hydroxide, this trend also shown in Fig.2. NaOH used as base for the removal of HCl from the reaction mixture during the hydrodechlorination of 2,4-DCP, the same as NH4OH. NaOH being a strong alkali also help for the elimination of Cl-atoms of 2,4-DCP during HDC reaction and its effect can be seen in the reduction of time for hydrodechlorination compared to as NH4OH.
Figure : The effect of different bases on the hydrodechlorination of 2,4-Dichlorophenol over Pd/C at 30°C
As a base Et3N directly react with HCl to form triethylamine-HCl in the HDC of 2,4-DCP, so the poison of HCl to the catalyst removed[1-5]. Triethylamin-HCl produced in the HDC could be used as a modifier which influenced the reaction rate by mediating the substrate-catalyst intreractions[6]. In some literature was found that amines and onium salts could change the reactivity and selectivity of Pt/C, Pd/C and Raney Ni in HDC reactions of haloaromatics[6,7]. Et3N can provide an electron pair to the surface and active sites of the metal catalysts such as Pt, Pd, Rh and Ni[8,9] because Et3N have an electron pair on N-atom. Due to this property, there might be a chance that the surface properties of the Pd catalyst changed which might influence the reactivity and selectivity toward ortho and para positioned atoms of 2,4-DCPin the hydrodechlorinaion reaction.
When Hydrazine used as a base, the dechlorinaion reaction was faster than other bases (NH4OH, NaOH and Et3N) as shown in the fig. When HCl react with hydrazine, hydrazine-HCl produced in the hydrodechlorination of 2,4-DCP. Hydrazine-HCl could be used as a modifier to the catalyst which possibly affects the rate of reaction.
Figure : The effect of different amount of hydrazine on the hydrodechlorination of 2,4-Dichlorophenol over Pd/C at 30°C
Dual character of Hydrazine
Although external hydrogen provided as a reducing agent but from the Fig.2, it was clear that in the presence of hydrazine as a base, the dechlorination reaction was faster instead of other bases. The basic role of hydrazine was to neutralize the HCl formed during the reaction but there was a possibility that hydrazine act as a reducing agent in the hydrodechlorination process as well. To prove this point different experiments were performed. Table.5 showed the results of these experiments. 2-CP and 4-CP were the intermediate products which were further converted to phenol.
Table 5: The Hydrodechlorination of 2,4-DCP over 5% Pd/C catalyst with Hydrazine(as a base) in liquid phase
Hydrogen source
Time (min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
Molecular Hydrogen
0
10
20
30
40
100
58.83
31.01
1.26
0
0
2.58
7.11
1.15
-
0
4.97
5.48
2.08
-
0
33.62
56.40
95.16
100
Hydrazine
0
15
30
50
70
90
120
150
100
68.17
32.66
15.26
8.19
2.86
1.50
0
0
6.59
7.82
3.70
1.12
1.66
0.38
-
0
1.35
5.31
2.18
1.03
-
-
-
0
23.89
54.21
78.86
89.66
95.48
98.12
100
Reaction conditions: 300 ml solution containing 2,4-DCP (3.00 mmol, 489 mg), 35 % Hydrazine (12 mmol, 1097.16 mg ) and 5% Pd/C (225 mg), Temperature 30 °C, Hydrogen gas flowrate 100 ml H2 min-1
Figure. 4 showed that when external hydrogen used as a reducing agent the hydrodechlorination reaction was faster than that in which no external hydrogen provided as a reducing agent. It means that when no external hydrogen provided then hydrazine act as a source of hydrogen. Although the reaction rate was slow but the reaction completed after 150 min. In the literature it has been reported that hydrazine act as an effective reducing agent for hydrodechlorination [10-11]. This trend was coincident with the results reported in literature [10-11].
Figure :
Hydrazine for chemical recovery
Table 6: The hydrodechlorination of different concentrations of 2,4-DCP over 5% Pd/C catalyst with Hydrazine(as a base) in liquid phase
Concentration of 2,4-DCP(mmol)
Time (min)
Product Distribution (%)
2,4-DCP
2-CP
4-CP
Phenol
3
0
10
20
30
40
100
58.83
31.01
1.26
0
0
2.58
5.48
1.51
-
0
4.97
7.11
2.08
-
0
33.62
56.40
95.16
100
6
0
10
20
30
40
100
52.44
9.67
0.69
0
0
0.24
0.36
-
-
0
3.93
4.86
0.69
-
0
43.39
85.11
99.09
100
9
0
10
20
30
40
100
23.44
43.41
15.53
0
0
0.95
-
-
0
7.88
2.78
0.85
0
67.73
93.71
98.49
100
Reaction conditions: 300 ml solution containing 2,4-DCP (3.00 mmol, 489 mg), 35 % Hydrazine (12 mmol, 1097.16 mg ) and 5% Pd/C (225 mg), Temperature 30 °C, Hydrogen gas flowrate 100 ml H2 min-1
Figure :
